Re: Question about the XEN platform pci

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: "karim.allah.ahmed@gmail.com" <karim.allah.ahmed@gmail.com>
Cc: xen-devel@lists.xenproject.org, "Wu, Bob" <Bob.Wu@emc.com>
Subject: Re: Question about the XEN platform pci
Date: Wed, 13 Apr 2016 05:55:38 -0400	[thread overview]
Message-ID: <20160413095537.GD2133@localhost.localdomain> (raw)
In-Reply-To: <CAOTdubsXgaYwHbddKYtvfk_RofobuZWP==H2Dwf-5DGrasgHpw@mail.gmail.com>

On Wed, Apr 13, 2016 at 09:00:18AM +0200, karim.allah.ahmed@gmail.com wrote:
> On Tue, Apr 12, 2016 at 5:45 PM, Konrad Rzeszutek Wilk
> <konrad.wilk@oracle.com> wrote:
> > On Tue, Apr 12, 2016 at 05:33:47PM +0200, karim.allah.ahmed@gmail.com wrote:
> >> The INTx interrupt of this platform device can be used by Xen in HVM case to
> >> notify the guest of pending events in the event channel. However that's usually
> >> not used in favor of vector callbacks support in Xen where a vector is injected
> >> directly to the vCPU bypassing LAPIC.
> >>
> >> (that said, the platform-pci driver in linux is actually broken when vector
> >> callbacks are not used anyway)
> >
> > Oh? Is there an report/bug somewhere?
> >
> 
> I'm not sure if it's reported as a bug somewhere or not.
> 
> I've always assumed that INTx is deperecated and vector callbacks are the one
> that's supported that's why I never tried to fix it, in addition I never tried
> to reproduce it I just looked at the code and it seemed a little bit off as
> explained below.
> 
> Mainly xenbus_init is called during postcore_initcall which will eventually try
> to read a value from XenStore and will get stuck on read_reply at xenbus
> forever since the platform driver is not probed yet and its INTx interrupt
> handler is not registered yet which basically means that the guest can not be
> notified at this moment of any pending event channels and none of the per-event
> handlers will ever be invoked (including the XenStore one) and the reply will
> never be picked up by the kernel.

Argh, well that is not good. Also - thanks for reporting this.

No simple ideas come to my mind on how this can be fixed - unless we
move the xenbus_init and its driver past the platform-pci init?

Or if platform-pci init code ends up being called earlier?

> 
> The exact stack where things get stuck during xenbus_init:
> 
> -xenbus_init
>  -xs_init
>   -xs_reset_watches
>    -xenbus_scanf
>     -xenbus_read
>      -xs_single
>       -xs_single
>        -xs_talkv
> 
> > Thanks!
> >>
> >> I also think that the grant-table lives on this PCI device MMIO BAR (?!)
> >
> > The area may be usurped for grant-table as the OS won't touch that memory
> > area (it after all belongs to the device).
> >>
> >> If you looked at hw/i386/xen/xen_platform.c in QEMU source , you will get a
> >> general idea what this device is supposed todo (like logging to syslog stuff
> >> for example).
> >>
> >> That said the platform device is really not fully utilized anyway in Linux.
> >>
> >> On Tue, Apr 12, 2016 at 4:09 PM, Konrad Rzeszutek Wilk
> >> <konrad.wilk@oracle.com> wrote:
> >> > On Tue, Apr 12, 2016 at 02:19:48AM +0000, Wu, Bob wrote:
> >> >>
> >> >> Really thanks for your reply.
> >> >
> >> > Hey!
> >> >
> >> > CC-ing Xen-devel back on. Please do not drop it and please don't
> >> > top-post.
> >> >>
> >> >> Can you explain a little more?
> >> >
> >> > I am not sure what you want me to explain. Perhaps if you
> >> > read http://xenbits.xen.org/docs/unstable/misc/hvm-emulated-unplug.html
> >> > it may become clearer?
> >> >
> >> >> Is the xen platform pci driver the only purpose for telling QEMU  that don’t emulate the IDE driver?
> >> >
> >> > And network.
> >> >> I think it can be done by a simple way, but don't need use this huge platform driver.
> >> >
> >> > ?
> >> >>
> >> >> I guess this is for PCI pass through in XEN HVM mode, but don't sure.
> >> >
> >> > No.
> >> >>
> >> >> Thanks,
> >> >> Bob
> >> >>
> >> >>
> >> >> -----Original Message-----
> >> >> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@oracle.com]
> >> >> Sent: 2016年4月11日 22:24
> >> >> To: Wu, Bob
> >> >> Cc: xen-devel@lists.xen.org
> >> >> Subject: Re: [Xen-devel] Question about the XEN platform pci
> >> >>
> >> >> On Fri, Apr 08, 2016 at 08:52:08AM +0000, Wu, Bob wrote:
> >> >> >
> >> >> > Sorry bother, I read the XEN source code recently, and found the XEN
> >> >> > platform PCI code under drivers/xen/platform-pci.c, and I can't fully under this driver's affect, can anybody explain a little for me?
> >> >> >
> >> >> > Is the platform PCI driver for PV-split-PCI-driver-model such as the pci-frontend/pci-backend? or for PCI pass-through model? Or for other purpose?
> >> >> > I saw the xenbus_pcifront_driver/ xenbus_xen_pcibk_driver are registered on XENBUS, so I guess the platform-PCI-driver is not for PV PCI driver.
> >> >> >
> >> >>
> >> >> It is for the QEMU driver. To tell QEMU to stop emulating the IDE/network.
> >> >>
> >> >> > Really thank you for your replay.
> >> >> >
> >> >> > Thanks,
> >> >> > Bob
> >> >> >
> >> >>
> >> >> > _______________________________________________
> >> >> > Xen-devel mailing list
> >> >> > Xen-devel@lists.xen.org
> >> >> > http://lists.xen.org/xen-devel
> >> >>
> >> >
> >> > _______________________________________________
> >> > Xen-devel mailing list
> >> > Xen-devel@lists.xen.org
> >> > http://lists.xen.org/xen-devel
> >>
> >>
> >>
> >> --
> >> Karim Allah Ahmed.
> 
> 
> 
> -- 
> Karim Allah Ahmed.

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